In reprographic devices, e.g. ink jet printers and so forth, it is commonplace to apply half-toning to the image data that is to be reproduced. Half-toning is a reprographic technique in which a continuous tone image (contone image) is represented using dots, notably using dots in cells, each cell representing a small region of the image. The number of dots used in a cell is varied depending on the content of the contone image so that different densities in the contone image can be represented, notably by using different numbers of dots in the cells of the reproduction image. Instead of using different numbers of dots in a cell, half-toning may use dots of differing sizes to allow different intensities in the contone image to be represented.
Typically the number of different values that can be represented by the dots in the cells of the reproduction image is much smaller than the number of different densities that may be present in the contone data. For example, nowadays typical print data is represented using 8-bits-per-pixel and the half-toning process may determine whether to fire ink (binary “1” level) or not to fire ink (binary “0” level) for a cell in the printed image that represents a given pixel in the input image data (so only two different density levels can be represented in the half-toned image, or “reproduction image”). In other cases plural ink drops may be fired for a single cell in the printed image: for example, 0 to 4 ink drops may be fired for each cell, allowing 5 different density levels to be represented in the reproduction image. Nevertheless, because the number of levels used to represent cells of the reproduction image is much less than the number of bits-per-pixel used to represent cells/pixels in the input image data, the density level assigned to a cell of the reproduction image will tend to depart from the true value of the density of the image at this location in the input image.
So, the half-toning process produces errors in the reproduction image, that is, the reproduction image is not totally faithful to the original contone image. The difference between the density at a particular region in the contone image and the density of the corresponding cell in the reproduction image is often termed quantization error.
Nowadays it is customary for reprographic devices to have a special black path, that is, to process image data relating to black lines in a separate processing pipeline from other image data, notably so that the black line data is not subjected to half-toning. The image data that is not processed in the special black path is still subjected to half-toning.
It is usual to design the half-toning process that is applied to the image data outside the special black path in a manner that will enhance the uniformity of area fills. This is because the most visible quality defects in a reproduction image tend to be non-uniformities in area fills.